feat(branding): force color mode = standard look; hide overridden theme controls

When a force light/dark lock is active it now means "use the clean standard
look": applyForceColorMode also resets typography & style (fonts, size, corner
radius, shadow, background pattern) to defaults — on top of the surface/text
palette it already swapped — so those settings genuinely don't apply while the
lock is on. Accent brand colours and the structural cards (header/controls/
gallery layout/hero divider) are preserved. Override-only: the saved theme keeps
the admin's custom values, so turning the lock off restores them.

In the theme customizer, when a force mode is active, hide the now-dead controls
to avoid confusion — the per-theme Color Mode picker, the Surfaces + Text colour
pickers, and the whole Typography & Style card — and show an explanatory note.
The Force picker itself and the Accent pickers stay visible. i18n en/de.

This pairs with the admin dark-mode fix: admin surfaces follow the `.dark` class
which AdminDarkModeContext drives from the force lock, so force is respected
end-to-end.

frontend/src/i18n/locales/de.json

@@ -1956,6 +1956,7 @@
     "forceColorModeNone": "Kein Zwang (Benutzerauswahl)",
     "forceColorModeDark": "Dunkelmodus erzwingen",
     "forceColorModeLight": "Hellmodus erzwingen",
+    "forcedStandardLookNote": "Ein erzwungener Farbmodus ist aktiv — die Galerie verwendet das Standard-Hell-/Dunkel-Design. Farbmodus, Flächen- & Textfarben sowie Typografie & Stil werden hier ausgeblendet, da sie bei aktiver Sperre nicht greifen. Akzentfarben gelten weiterhin. Setze „Erzwingen“ unten auf „Keine Erzwingung“, um sie anzupassen.",
     "colorGroupSurfaces": "Oberflächen",
     "colorGroupSurfacesHelp": "Die neutralen Ebenen hinter Ihrem Inhalt. Hintergrund liegt am weitesten zurück; Oberfläche und Erhöht stapeln sich darüber.",
     "backgroundColorHelp": "Die Seite selbst — Hintergrundfarbe jeder Galerie, Admin-Seite und CMS-Seite.",

frontend/src/i18n/locales/en.json

@@ -1545,6 +1545,7 @@
     "forceColorModeNone": "No force (user choice)",
     "forceColorModeDark": "Force dark",
     "forceColorModeLight": "Force light",
+    "forcedStandardLookNote": "A forced color mode is active — the gallery uses the standard light/dark look. Color mode, surface & text colors, and Typography & Style are hidden here because they don’t apply while the lock is on. Accent colors still apply. Set Force below to “No force” to customize them.",
     "colorGroupSurfaces": "Surfaces",
     "colorGroupSurfacesHelp": "The neutral layers behind your content. Background sits furthest back; Surface and Elevated stack on top.",
     "backgroundColorHelp": "The page itself — body background of every gallery, admin page and CMS page.",